This is a multi-center, double-blind, randomized, parallel group, add-on study in patients with complex partial seizures with or without secondary generalization which have not been satisfactorily controlled with either carbamazepine or phenytoin monotherapy. The primary objective of this is to compare the safety and efficacy of tiagabine HCL with carbamazepine and phenytoin when administered as the first add-on treatment for patients with complex partial seizures with or without secondary generalization who are not controlled with carbamazepine or phenytoin monotherapy. The GCRC psychometrist is utilized for the neuropsychological assessments.